In the installer, you will have the option to customize. It will be a button on the bottom at one of the stages.
When customizing, only include essential system files (dump all the extra fonts, applications, and drivers) and you
should be ok with just one disc. I highly suggest downloading Carbon Copy Cloner (
Link) and cloning your current drive to an external if possible, just in case it goes pear shaped you'll be able to have a working computer again. You may be able to borrow an extra drive if you don't have one.
Or, if you're super concerned, just try deleting a whole bunch of files you don't need and run all your diagnostics (download Cocktail or something similar and run all the scripts). Another option is to get a copy of iDefrag and run that, it will give you some new life. But the biggest performance increase you'll see will be creating more open space on your drive, so anything non-essential should be deleted whatever route you take.
Good luck!
EDIT: itunes is not part of iLife, though it may not install in the minimal, but you can then download it from Apple. Again, note that you SHOULD be able to do an install with only one disc, I think I've done it before, but I can't be 100% sure, so definitely back-up if possible...
EDIT 2: If you try this route and it demands the second disc after the restart (it will automatically restart at the end of disc one), try hard shutting down (hold down the power button until it powers down) and then reboot. If it prompts you again, you may be out of luck, though I'd bet if you google around you'll be able to find instructions on how to complete it in single-user mode without the disc...